Scientists have uncovered more evidence of a "switch" which allows breast cancer to grow and spread.

The finding, by US researchers, could lead to drugs precisely targeted to stop this process in its tracks.

Mice genetically altered to produce larger quantities of a chemical called COX-2 had faster-growing and spreading breast cancers.

Drugs that "inhibit" COX-2 - from the aspirin family - could have a role fighting breast cancer, say experts.

The research was carried out at the University of Connecticut, and published in the journal Proceedings of the National Academy of Sciences.

One of the key factors that allows a tumour to grow is whether it has sufficient blood supply to support its new size.

Many tumours can harness chemical pathways that prompt the body to create a web of new blood vessels around the cancer, a process called angiogenesis.

COX-2, and another chemical linked to it, called prostaglandin E2 (PGE2), are already under suspicion for having a role in this process.

Altered mouse

If this role is proven, there are already drugs available which could interfere with this process, and perhaps improve the chances of patients with breast cancer, which has become the most common cancer in women in the UK.

Dr Timothy Hla, who led the study, created a genetically modified mouse which produced more COX-2 in its breast tissue - in theory producing the perfect environment for a breast tumour to create the necessary blood vessels to allow growth.

This was what they found - blood vessel density increased prior to visible tumour growth in the mouse breast tissue, and during progression, the density of the blood vessels increased at an exponential way.

When drugs called COX-2 inhibitors - designed to interfere with the workings of this chemical - were added to the mix, tumour growth slowed and blood vessel density decreased, pointing again to the role of COX-2 in the process.

Household medicines

Several commonplace household drugs have been shown to have an inhibitory effect on COX-2 - including aspirin and ibroprofen - which in theory could be used to prevent or hold back breast tumours.

However, doctors say they could create a more specific drug which avoids some of the dangerous side-effects of long-term painkiller use.

Dr Henry Jabbour, from the MRC Human Reproductive Sciences Unit in Edinburgh, is interested in the potential of these drugs to tackle a variety of cancers.

He said: "There have already been trials in colon cancer, but it possible they could be effective in other cancer types.